Fig. 1. Reference plane determination and measurement parameters. (A) Horizontal-sectional slice, (B) Sagittal-sectional slice, (C) Coronal-sectional slice, (D) Measurement parameters in the sagittal section, (E) Measurement parameters in the coronal section of the mesial root, (F) Measurement parameters of the coronal section of the distal root. The green line represents the coronal plane; the blue line represents the sagittal plane; and red represents the horizontal plane, which is parallel to the patient’s occlusal plane. The purple line represents the border of the inferior alveolar nerve. The yellow dotted line indicates the measured parameters. MD-SS: mesial-distal socket size; BL-SS: buccal-lingual socket size; RF-I: root furcation fornix to inferior alveolar nerve distance; IRB2/IRB4: the interradicular alveolar bone thickness at 2/4 mm apical from the furcation fornix; IRBA: the interradicular alveolar bone thickness between the mesial and distal root apex; MRA-I/DRA-I: mesial/distal root apex to the inferior alveolar nerve distance; MR-B/MR-L: thickness of the buccal/lingual bone of the mesial root at 2 mm apical from the alveolar crest; DR-B/DR-L: thickness of the buccal/lingual bone of the distal root at 2 mm apical from the alveolar crest.